The invention relates to a piston-ring system and, more particularly, a plastic ring system for sealing a piston-like, cylindric, refrigerator displacer reciprocably in a cylindric housing.
Some refrigerators produce low-temperatures by reciprocating a piston-like displacer in a gas-containing housing in a thermodynamic cycle. (See, for example, U.S. Pat. No. 2,906,101.) A single-stage refrigerator of this type generally has a cylindric housing chamber for a single, cylindric displacer that is coupled to an electric or pneumatic drive for reciprocation therein. The chamber is alternately connected, in a particular manner, with high- and low-pressure, working-gas sources. The working gas flows through a regenerator in the displacer before and after expansion by the reciprocation of the displacer for the thermodynamic cycle (Stirling cycle, Gifford/McMahon cycle, etc.) of the refrigerator. As a result, heat is abstracted from a specific region of the chamber. With a refrigerator of this type having two such stages and helium as the working gas, temperatures of 10 K or lower can be produced.
A necessary, critical aspect, which is of decisive importance for the operation of such a refrigerator, is lubricant-free sealing of the displacer relative to the chamber wall of the housing which surrounds it. On the one hand, the seal must satisfy stringent tightness requirements to assure the flow of the working gas through the regenerator and, thus, the refrigerator. On the other hand, the seal should produce as little friction as possible with the housing-chamber wall during displacer reciprocation, because such friction would be accompanied by evolution of heat which is particularly undesirable in a refrigerator.
Because of their good antifriction properties, plastics and, particularly, polytetrafluoroethylene (hereinafter PTFE) have gained wide acceptance as sealing materials. However, a drawback of PTFE is its highly-nonuniform and irreversible heat-shrinking behavior. Even rings made from one and the same block of PTFE material shrink differently. This drawback is particularly manifest in a PTFE sealing ring for refrigerator displacer because of the very-considerable temperature differences it experiences.
Nevertheless, a PTFE sealing-ring system for a reciprocating-displacer refrigerator is known from U.S. Pat. No. 4,355,519. It consists of a PTFE sealing ring held in a radial groove in the displacer by an axially-acting spring ring. The sealing ring is radially split in a section which is axially Z-shaped, in other words, Z-shaped perpendicularly to the plane of the sealing ring which is normal to the axis of the sealing ring. The resulting, overlapping portions of the split in the sealing ring are intended to compensate for circumferential length changes (e.g., shrinkage) from temperature fluctuations or wear. One disadvantage of this design is its complexity. The axially-acting spring ring, which reacts from one axial-end wall of the groove, cannot be split, and the displacer must be, therefore, of two-part construction to permit mounting the spring ring. Moreover, even slight shrinkage of the sealing ring material impairs the integrity of the seal, because the axially opposite ends of the Z-shaped split then open, and these communicate through the groove radially inward of the sealing ring.
Another split, possibly-plastic, piston-sealing ring is known from U.S. Pat. No. 3,455,565. Its split is arcuate, preferably circularly, in the plane of the ring. This ring has the drawback that its radial thickness changes in proximity to the split when the material shrinks. This has the effect that the radially outward forces of a spring inside the sealing ring for this are no longer uniform around the periphery of the ring. Two such rings, axially contiguous but with circumferentially-offset splits, are also disclosed.